An Experimental Study on the SRPC and CSA Cement Systems ... · An Experimental Study on the SRPC...

An Experimental Study on the SRPC and CSA

Cement Systems Based on Flyash and Anhydrite

K. Thiruppathi

a*, S. Barathan

b, G. Sivakumar

b

(Collabration with Structural and Civil Engineering Department, Annamalai University) a*

Department of Physics, Valliammai Engineering College, SRM Nagar, Kattankulathur, Chennai, Tamil Nadu, India 603 203. b Department of Physics, Annamalai University, Annamalai Nagar, Tamil Nadu, India 608002.

Abstract—

This research paper deals, calcium sulfoaluminate

based cement (CSAC) and Sulphate Resisting Portland Cement

(SRPC) with fly ash (FA) was used and the effect of fly ash

(which contains calcium sulfoaluminate) on the properties of the

systems was studied with SRPC. Fly ash (FA), anhydrite (ANH)

and flue gas desulfurization gypsum (FGDG) were used to

develop appropriate addition systems, the hydration of which

was studied. The compressive strength and the setting time

properties of cements were tested. The results suggest that the

use of fly ash (which contains calcium

sulfoaluminate) accelerates

the formation of a strong ettringite-rich matrix that firmly

accommodated unreacted fly ash particles, and contributing to a

denser microstructure. At CSAC in a given sulphate content, the

anhydrite was shown to be favourable in terms of the setting

times, heat patterns (higher) compared to SRPC blended with FA

based formulations however in strength development in reverse

order. The Scanning Electron Microscope (SEM), Thermo

gravimetric (TG) analysis and XRD also shows similar results.

Abbreviations--

E, Ettringite; A, Anhydrite; Y, Ye'elimite; S,

Stratlingite; G, Gypsum;

Keywords—

Sulfoaluminate, Ettringite, Fly ash, Hydration, C-

S-H

I.

INTRODUCTION

The mixing of cementitious materials and additives with Portland cement (PC) is a well established approach to reducing the CO2 emissions associated with the energy-intensive manufacture of cement. At present around 3.5 billion tonnes of PC is globally manufactured every year and it is estimated that the embodied CO2 (eCO2) for PC production is approximately 900 kg of CO2 per tonne of PC produced [1] and [2,3]. The way to reduce the CO2 of cement is through the use of non-PC based systems as the binding ingredient. The calcium sulfoaluminate cement (CSAC) and fly ash (which contains calcium sulfoaluminate) with sulphate resisting Portland cement (SRPC) which have a lower emission of CO2 than PC, have been used in the present work [7]. The emission of CO2 of a typical pure CSAC and calcium sulfoaluminate rich fly ash with SRPC, consisting of ye'elimite, belite and aluminoferrite, is approximately 590 kg/t. This represents a reduction in emission of CO2 approximately 30% to 35% when compared to PC.

The early hydration product of CSA rich fly ash with SRPC and CSAC is ettringite (3CaO.Al2O3.3CaSO4.32H2O) which forms as primary product (within 24 h) as prismatic needles. The formation of ettringite in the presence of sufficient calcium sulfoaluminate in SRPC with fly ash and CSAC which can be either presence of the ye'elimite-rich clinker (during manufacturing) or added in the raw meal intergrinding process. The optimum quantity of calcium sulfoaluminate for dominant formation of ettringite depends on several parameters i.e. the ye'elimite, the calcium sulfoaluminate content and their respective molar ratios [8]. If there is deficiency in calcium sulfate, then there is a tendency for monosulfoaluminate to form; whereas an excess of calcium sulfate may lead to unstable expanding systems [9]. Other products of the hydration of CSA rich fly ash with SRPC and CSAC are mainly aluminate hydrates and calcium silicate hydrates. Unreacted ye'elimite is also typically present. Strengths of CSAC and CSA rich fly ash with SRPC based cements may reach over 40 MPa at 28 days [10] and SRPC+30% FA over 50 MPa at 28 days and they have been reported to exhibit very good resistance to aggressive environments, particularly to sulfate environments [11] and [12]. This is because aluminate-based phases are bound as sulfoaluminates at early stages of hydration and these are not available for reaction with external sulphate agents to form expansive ettringite.

The aim of this paper to investigate the influence of two main aspects revolving around the optimization of fly ash with SRPC hydration and performance of CSAC based cement systems. The second aspect covered the development of a binary system through the use of a low emission CO2 addition, particularly fly ash, which could provide a better balance between performance and emission of CO2 with SRPC.

A. The calcium sulfoaluminate

There is a considerable experimental activity on development of CSAC and CSA rich fly ash blended SRPC that have been focusing on the use of anhydrite as the calcium sulphate source in the CSA system. Extensive work in the last decades is encapsulated in review articles[13,14]. Indeed, consensus indicates that anhydrous calcium sulfate is the predominant and preferable calcium sulfate source within the system. The use of other forms of calcium sulfate, particularly hemihydrate, however, is not fully documented so as to offer justifications of any cost-based, environmental-based or performance-based advantages associated with anhydrite preference in CSAC. Moreover, such lack of available data

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does not provide a clear indication of the influence of the hemihydrates of calcium sulfate on the hydration of CSAC. The CSA rich fly ash with SRPC also reported and compared with CSAC cements results. A more detailed look is therefore required at this type of calcium sulfate.

The intrinsic properties of both hemihydrate calcium sulfate and anhydrite are compared and established [15,16]. Setting times of the hemihydrate are known to be considerably short due to its high solubility (typically in the range of 7.5–9.5 g/l) and its reactivity, as opposed to that of anhydrite (approximately 3–4 g/l) [17]. Eqs. (1) and (2) suggest that during the reaction of both calcium sulfate forms with ye'elimite, the quantities of ettringite and Al(OH)3 formed are comparable. The only parameter that varies is the amount of water needed for complete phase formation.

Ye'elemite + anhydrite:

4CaO.3Al2O3.SO3 + 2(CaO.SO3) + 38H2O →

6CaO.Al2O3.3SO3.32H2O + 2(Al2O3.3H2O)----------------(1)

Ye'elemite + hemihydrate:

4CaO.3Al2O3.SO3 + 2(CaO.SO3.0.5H2O) + 37H2O →

6CaO.Al2O3.3SO3.32H2O + 2(Al2O3.3H2O)--------------- (2)

Based on the available literature and data, it is therefore necessary to distinguish the beneficial characteristics offered by each form of calcium sulfate when incorporated in CSA system.

B. The use of fly ash in SRPC

Although the eCO2 associated with the use of a fly ash in SRPC/calcium sulfate system may be lower than that of PC, potentially greater savings may be achieved based on the development of binary fly ash-based SRPC systems with maintained performance properties.

The CSA is formed as per the equation

3CaCO2 + 3Al2O3 + CaSO4.2H2O → 4CaO.3Al2O3.SO3 + 3CO2 + 2H2O------------(3)

By-products from coal combustion plants are associated with almost zero eCO2, whilst they may provide microstructural and mechanical advantages to cementitious systems when incorporated at optimum percentages. One advantage is the pozzolanic reaction. The use of low eCO2 pozzolanic by-products – particularly fly ash – in the SRPC system, may instigate reaction with Ca(OH)2 yielding from belite hydration in SRPC thus providing additional C–S–H gel. Previous studies on compressive strengths of SRPC/fly ash blends suggest a increase in strengths when fly ash contents are 30% [18]. However, there is still limited understanding and lack of data on the hydration mechanisms of such systems. Given this, and by considering the advantageous effect of particle packing that fly ash may potentially provide when acting as a low-eCO2 filler, then it is possible that a more sustainable system may be developed whilst maintaining its mechanical and microstructural properties[19].

II. MATERIALS AND METHODS

The materials used in this study are shown in Table 1. Particle size distribution was determined using a Malvern Mastersizer 2000 laser diffraction equipment. Anhydrite (ANH), plaster (PL) and flue gas desulphurization gypsum (FGDG) were used as the calcium sulfate sources in the CSAC system.

Table 1. Materials used in the research.

Material Abbreviation Particle density (kg/m3)

Mean diameter size (μm)

Particle size distribution (μm)

d10 d90

Sulphate Resisting Portland cement

SRPC 2780 25.1 2.1 64.6

Calcium sulfoaluminate

cement CSAC 2792 25.4 2.3 64.9

Fly ash, category N to BS EN 450-

1:2012 [18] FA 2293 34.6 2.6 81.8

Sulphate Resisting Portland cement

with fly ash SRPC+FA 2653 30.6 2.5 75.2

Calcium sulfate: flue gas

desulfurization gypsum

FGDG 2525 47.6 8.9 118.2

Calcium sulfate: gypsum plaster

PL 2650 26.5 3.4 86.2

Calcium sulfate: anhydrite

ANH 2950 23.5 2.1 42.9

The ye'elimite content in the CSAC clinker was found to be 65% and the belite content was 20%, although less amount of calcium sulfate was detected. To confirm the sulfate type in FGDG, PL, and ANH, TG analysis (30 °C to 350 °C at a rate of 10 °C/min) was conducted and mass losses of 5.23% and 5.06% respectively were obtained in the range of 140–145 °C. less mass loss was observed in ANH. The ye'elimite content in the fly ash doped SRPC was found to be 30% to 35% [20].

The hydration processes of the systems, cement pastes were prepared at a w/c ratio of 0.4 and cured in a 25 °C water-curing tank until age of testing. TG and XRD analyses were performed after 1, 3, 7 and 28 curing days and SEM images of the pastes cured for 28 days were obtained, assuming that this period was adequate for allowing full formation of all hydration products. Acetone was used to stop the hydration of the cements.

Setting times of the pastes were determined in accordance to BS EN 196-3:1995 [21]. Heat of hydration was determined using a Wexham development JAF conduction calorimeter. Mortar samples were prepared to assess the mechanical properties and dimensional changes of the combinations. The compressive strength was conducted in accordance to BS EN 196-1:1995 [21] and the dimensional changes were monitored on air-cured mortar samples stored in conditioning chamber (maintained 25 °C, 60% RH) at 1, 7, 28 and 90 days of age.

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For the development of appropriate CSAC/calcium sulfate combinations, and percentage of fly ash doping with SRPC the following criteria were taken into consideration:

•Achievement of a cement strength class equivalent (or higher) to that of a 42,5N (or R) conventional cement as defined in ASTM Committee C-1, 1987[23]

•A minimum content of calcium sulfate in the system to ensure ettringite formation and avoid monosulfoaluminate formation by using stoichiometric approach.

To examine the influence of the type and amount of the FGDG,ANH and FA sources on the mechanical properties and

dimensional stability of the system, combinations were prepared at varying FGDG,ANH contents, i.e. CSAC/FGDG: 100/0, 80/20; 70/30; CSAC/ANH:80/20; 70/30 and SRPC/FA : 70/30 as shown in Table 2.

Table 2.Mix proportions and compressive strength development of combinations used.

Cement/combination Notation

SRPC CSAC FGDG PL ANH FA

Compressive strength N/mm2

at day

1 3 7 28 90

CSAC

– 100 – – – – 32 40.7 55.2 70.2 75.4

CSAC/FGDG

CSAC/ANH

FGDG1 – 70 20 – – – 22.3 29.8 34.6 40.7 45.6

FGDG2 – 80 30 – – – 25.3 33.4 43.6 58.1 60.2

ANH1 – 70 – – 20 – 20.9 26.3 35.5 43.2 45.7

ANH2 – 80 – – 30 – 28.2 37.4 48.2 64.2 63.2

SRPC SRPC1 100 – – – – – 21.58 32.47 43.56 49.74 57.53

SRPC SRPC2 70 – – – – 30 18.44 26.39 49.98 57.48 66.34

For the development and selection of appropriate

CSAC/anhydrite and SRPC/FA combinations (denoted in

Table 2), FA was introduced at contents of 5%, 10%,15%,

20% and 30% by mass of total cement, among them 30%

was the best one, in the same way CSAC/anhydrite, whilst

maintaining the CSAC/anhydrite ratio that satisfied all the

criteria previously stated. The particular percentage range of

FA was selected based on determining approximately the

minimum amount of Ca(OH)2 likely to be formed from

belite hydration so as to promote pozzolanic reaction.

To establish the minimum calcium sulfate content for

sufficient ettringite formation, a stoichiometric approach

was used. For complete ettringite formation in both cases,

the molecular mass ratios of calcium sulfate to ye'elimite

needed to be considered based on Eqs. (1),(2) and (3). Based

on the chemical composition of the as-received high-

strength CSAC, the pure ye'elimite content was 71% and no

calcium sulfate was added and/or interground in advance

during manufacturing. It was assumed that the full amounts

of added calcium sulfate reacted solely with ye'elimite,

therefore, the minimum calcium sulfate content required to

promote ettringite formation and avoid monosulfoaluminate

formation was calculated as 0.477 × 0.71 ≈ 34% for the

hemihydrate and 0.448 × 0.71 ≈ 32% for the anhydrite,

respectively. The two contents were then considered as the

limits for defining chemically stable CSAC/ANH and

CSAC/hemihydrate combinations.

III. RESULTS AND DISCUSSION

A. Dimensional stability

Dimensional changes of CSAC/ANH and CSAC/FGDG and

SRPC/FA mortars at e shown in Fig. 1. It can be seen that

at below 30%, both FGDG and ANH-based formulations

exhibited similar and almost dimensionally neutral patterns

at early ages. These were followed by a slight shrinkage on

the region of 5–10 × 10− 4 on the 28th day, reaching the

maximum values on the 90th day. When FGDG and ANH

contents reached above 30%, expansion starts occurred to

both combinations with lighter cracks forming at a

characteristic of ettringite's instability when ye'elimite is

introduced in exceedingly high sulfate concentrations, so

that is not good proportion to discuss, it is neglected. Based

on these results, the value of 30% was considered as the

maximum content when selecting stable CSAC/ FGDG and

ANH combinations. Shrinkage patterns of CSAC/FGDG

were less than those observed CSAC/ANH at a given

proportion, with a difference ranging from approximately 40

to 160 × 10− 4 strains. This was because, for the calcium

sulfate to yield the same molecular weight of ettringite upon

their reaction with ye'elimite, the hemihydrate content

requirement was higher than that of anhydrite, according to

Eq. 1 and 2. Towards the 90th day of examination, slight

shrinkage was observed for both CSAC/ANH and

CSAC/FGDG samples contents less than 30%, probably due

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0

1

2

3

4

0 10 20 30 40 50 60 70 80 90

Y CSAC/FGDG 0% Y CSAC/FGDG 20% Y CSAC/FGDG 30%

Y CSAC/ANH 0% Y CSAC/ANH20% Y CSAC/ANH30%

Y SRPC/FA0% Y SRPC/FA30%

to a small degree of ongoing water evaporation occurred

after complete ettringite formation. In the same way

SRPC/FA contents less than 30%, slight shrinkage was

observed probably due to a small degree of ongoing water

evaporation occurred after complete ettringite formation.

Fig. 1. Dimensional changes of CSAC/FGDG, CSAC/ANH and SRPC/FA combinations at varying contents

Fig.2 The compressive strength of CSAC/FGDG, CSAC/ANH and SRPC/FA at varying limits of stability.

B. Compressive strength development

Compressive strength development of water-cured

CSAC/FGDG, CSAC/ANH and SRPC/FA mortars at

increasing days as shown in Table 2 and Fig.2.

The CSAC/FGDG, CSAC/ANH and SRPC/FA

combinations the results show that, regardless of the type

introduced, the compressive strength values tend to increase

at increasing ANH or FGDG and FA contents. The highest

strengths were observed in pure CSAC and SRPC/FA in

30% mortars. This is in coherence with previous work [22]

when considering 7-day and 28-day values. During the first

24 h of hydration, the formation of ettringite in the CSAC or

SRPC systems would be mainly responsible for the systems'

strength development as evidenced from TG, XRD and

SEM analyses in Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8,

Fig. 9, Fig.10 and Fig. 11. In pure CSAC, where the

ye'elimite content is highest compared to all other

formulations, the strength evolution at very early ages

Age in days

0

10

20

30

40

50

60

70

80

1 3 7 28 90

CSAC/FGDG 0% CSAC/FGDG 20% CSAC/FGDG 30% CSAC/ANH20%

CSAC/ANH30% SRPC/FA0% SRPC/FA30%

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would be dependent on the formation of other hydration

products, most probably C–A–H, Al(OH)3 and small

amounts of ettringite due to the absence of calcium sulfate.

The effect of this possible set of hydration products on the

strength development of pure CSAC could potentially be

greater than that of ettringite upon the CSAC/FGDG,

CSAC/ANH and SRPC/FA systems. In the similar way, the

strength development most probably C–A–H, Al(OH)3 and

small amounts of ettringite due to the absence of calcium

sulphate in SRPC systems. In SRPC/FA, where the

ye'elimite content is highest compared to all other

formulations, the strength evolution at very early ages

would be dependent on the formation of other hydration

products [23]. According to dTG curves in Fig. 4,

Fig. 7 and Fig. 10, the ANH and FGDG in the CSAC and

SRPC/FA systems did not appear to be fully depleted from

day 1 so as to give full amounts of ettringite and provide

high strengths — possibly due to low reactivity of the

calcium sulfate materials and fly ash. Complete ettringite

formation for most of the systems did not seem to occur at

least until the 7th day and this may have caused the systems

to exhibit lower 1-day strengths than CSAC and SRPC.

5 2θ 60 Fig. 3. X-ray diffractograms of70%CSAC/30%FGDG obtained at 1, 3, 7 and 28 days of hydration.

6 hour

10 hour

1 day

1 week

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Fig. 4. dTG curves and mass losses in 70%CSAC/30%FGDG obtained at 1, 3, 7 and 28 days of hydration.

Fig.5. SEM image of 70%CSAC/30%FGDG obtained at 28 days.

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Fig. 7. dTG curves of 70%CSAC/30%ANH at 1, 3, 7 and 28 days of hydration.

Fig. 8. SEM image of 70%CSAC/30%ANH at 28 days of hydration.

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Fig.

9.

X-ray diffractograms of70%SRPC/30%

FA

at 1, 3, 7 and 28

days of hydration.

Fig.

10.

dTG curves of 70%SRPC/30%FA

at 1, 3, 7 and 28

days of hydration.

5 2θ 60

9 hour

1 day

1 d

3 d

7 d

28d

E

E

EY

Y

E

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Fig.

11.

SEM image of

70%SRPC/30%FA

at 28

days of hydration

with EDX

The use of both FGDG and ANH in CSAC at a

proportion of 20% and 30% by mass of the combination

gave 28-day strength values (40.7, 43.2 N/mm2

and

58.1,64.2 N/mm2) and SRPC/FA of 30% (57.48 N/mm

2

meeting the target 40.5N/mm

2

(Fig.

2)

and no strength loss

was observed up to 90

days. Based on these results and by

considering the chemical and dimensional criteria discussed

in above section,

the selected CSAC/ 30% FGDG or ANH.

This particular content was sufficient to ensure complete

ettringite formation and avoid monosulfoaluminate

formation as in stoichiometric calculations. The SRPC

combination with FA at 30% is sufficient to develop

required strength at 28 days. In addition, the content was

such that did not appear to cause dimensional instability

according to Fig.1.

Compressive strength development combinations

showed

that within a maintained CSAC/ANH ratio,

CSAC/FGDG and SRPC/ FA at a 30% content gave an

increase in 28-day strengths compared to20%. Therefore the

chosen percentage formulation was the one consisting of

30% by mass of FA, FGDG and ANH as it met the target

strength class at maximum percentage of the addition.

C.

Hydration

The hydration processes of CSAC/FGDG, CSAC/ANH and

SRPC/FA mixes were investigated through TG, XRD and

SEM analyses and each system is discussed below.

CSAC/FGDG

X-ray diffractograms and dTG curves of the stable

CSAC/FGDG system are shown in Fig.

3

and

Fig.

4,

respectively. SEM images of 28-day sample are shown in

Fig.

5. Based on the diffractograms, the main hydration

product was ettringite, and unreacted ye'elimite, gypsum,

and

gehlenite peaks were also detected. Ettringite

corresponding XRD peaks and dTG curves occurred from

the first 24

h of hydration. SEM images showed a

homogeneous microstructure with rich amounts of needles

within a pore. The prismatic needles had a thickness of

approximately 1–2

μm and a length ranging from 40 to

80

μm.

Based on the dTG analysis, the corresponding

weight loss at 110–125

°C and the mass loss were

progressively increasing towards the 7th day. At slightly

higher temperatures (140–150

°C), an additional mass loss

was observed, which was assigned to the amounts of

gypsum formed in the combination and excess loss of water

content [24]. At 28

days, however,

overlapping occurred

between corresponding decomposition temperatures of

ettringite and gypsum.

The alumina hydrates were detected by TG

analysis with a mass loss progressively increasing with time

(4.1–5.2%) between 260

°C–270

°C,. This phase was not

detectable by XRD analysis due to its non-crystalline

structure.

CSAC/ANH

Hydration of CSAC/ANH as determined through

XRD, TG and SEM is shown in Fig.

6

and

7 and 8

respectively. XRD results showed that the crystalline

products of 70%CSAC/30%ANH were equivalent to those

of the other two systems, i.e. ettringite, unreacted ye'elimite

and gehlenite. Ettringite peaks as shown in Fig.6, became

progressively stronger

as with the other systems, reaching a

maximum on the 28th day of hydration. In this system, no

overlapping occurred between dTG gypsum peaks and

Ettringite

FA

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ettringite peaks, as calcium sulfate was provided solely in

the form of anhydrite. Consequently, no hemihydrate dTG

peaks were available and therefore quantitative dTG mass

losses were entirely ascribed to the amount of ettringite.

Towards the 28th day, ettringite mass loss was increasing

from approximately 10% to approximately 20.5%. An

increase in mass loss of Al(OH)3 was observed (almost 2%

increase) on the 28th day compared to day 1.

SEM images (Fig. 8) showed rich amounts of prismatic

ettringite needles within a pore, and no significant

difference in morphology and mineralogy was observed

compared to 70%CSAC/30%FGDG images.

70%SRPC/30%FA

The X-ray diffractograms, dTG curves at 1, 3, 7 and 28 days

and SEM images at 28 days for SRPC/FA of 30% are

shown in Fig.9, 10 and 11 respectively.

In the results, ettringite was formed from the first

day of hydration, with a 4.2% increase in mass loss when

reached the 28th day. In particular, the patterns obtained in

dTG analysis were similar as in 65%CSAC/35%ANH

combination. Crystalline products detected from XRD were

ettringite, unreacted ye'elimite, anhydrite and some

additional peaks were attributed to the presence of quartz

and mullite from the addition of FA and stratlingite

appearing onwards the 28th day. The formation of small

amounts of stratlingite may probably be a result of a

reaction between hydrated belite and aluminate-based

phases. The XRD peaks clearly indicates that different

phases present in the mixture during the 28

reaction. Al(OH)3 amounts were comparable to the other

combinations, having a dTG mass loss in the range of 4.1–

5.2% throughout the period of examination.

SEM images with EDX of SRPC/FA of 30% in

Fig. 11 showed a dense, homogeneous microstructure

consisting of rich amounts of prismatic ettringite needles

with unreacted FA particles. The microstructure observed

showed a synergistic effect between FA and ettringite. The

observed FA particles were immobilized and appeared to be

firmly wedged into spaces in-between the formed

sulfoaluminate phases, denoting that an effective void filling

had occurred.

By comparing XRD patterns of CSAC/FGDG,

CSAC/ANH and SRPC/FA it can be seen that common

hydration products detected were ettringite and unreacted

ye'elimite although Al(OH)3 could not be detected due to its

non-crystalline structure. Gypsum was detected at

CSAC/FGDF whereas anhydrite was detected in

CSAC/ANH. Ettringite XRD peaks were more intense in

CSAC/ANH than in CSAC/FGDG and SRPC/FA

particularly at an angle of 24.5°, probably due to the

overlapping peak of anhydrite. In the SRPC/FA little quartz

and mullite were additionally detected due to the FA

incorporated in the combination.

Isothermal conduction calorimetry

Heat patterns and output rates of CSAC/ANH,

CSAC/FGDG and SRPC/FA systems are shown in Fig. 12.

Fig. 12.Heat patterns and heat output rates of CSAC/FGDG, CSAC/ANH and SRPC/FA.

-2

0

2

4

6

8

10

12

0 5 10 15 20 25

CSAC SRPC CASA/FGDG20%

CASA/ANH20% CASA/FGDG30% CSAC/ANH30%

SRPC/FA30%

Hydration time (Hour)

Hea

t O

ut

pu

t W

/kg

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The heat pattern showed an initial shoulder peak

occurring in less than 2 h in very low rate, followed by a

maximum heat rate between the 3th and the 5th hour for all

combinations. Peaks were associated with the depletion of

calcium sulfate and the formation of sulfoaluminate and

aluminate hydrates. Output peak rates were higher in CSAC

and SRPC where as other combinations. The SRPC/FA

showed a heat pattern similar to that of others but with

lower maxima and equivalent peaks accelerates at

approximately 2 h compared with other maximas. All the

combinations showed the same tendency of exhibiting an

initial shoulder followed by a heat peak maximum. In

CSAC/FGDG, only one main peak was detected 2 and 4 h

earlier than those of SRPC/FA and CSAC/ANH,

respectively. This reflected the tendency of hemihydrate to

accelerate the formation of both sulfate hydrate and

sulfoaluminate hydrate phases simultaneously. The

particular peak was attributed to the formation of gypsum

and ettringite, both having occurred at the same period. A

smooth curve was also observed after a period of dormancy,

having a maximum heat output rate of less than 2 W/kg

between 13 h and 14 h, probably due to the precipitation of

further sulfoaluminate and aluminate-based phases in all

cases.

Initial and final setting times

Comparison of the initial and final setting times

between all examined cements is shown in Fig. 13

Fig. 13.Initial and final setting times of the investigated cements.

The initial setting of almost all cements and

combinations occurred at the beginning of their accelerating

heat pattern period and final setting times occurred before

the corresponding heat output maxima. In all combinations

regardless of the presence of FA, setting times were shorter

than those of a typical Portland cement-based combination,

mainly because of the high water demand during ettringite

formation. It is known that, the molecules of water are

attracted on the ettringite skeletal structure according to the

phase chemical composition. In contrast, the C–S–H gel is

associated with fewer molecules regardless of its

stoichiometric variations. Main factors influencing the water

adsorption rate in ettringite are mainly the morphology, the

crystalline structure, the phase size (larger than C–S–H) and

the interlocking effect between the compounds. Comparing

the behaviours of cement systems, the results suggest that

setting times would not normally raise concerns in

construction processes when considering transportation and

casting. By comparing, however the setting behaviours of

the CSAC/FGDG and SRPC/FA, it can be seen that there is

a notable difference in the initial and final sets. This may be

attributed to the reactivity and solubility of the two materials

which might have been affected by the presence of moisture

from the production process and/or any impurities present

[24].

The incorporation of FA in the SRPC/FA

combination caused a reduction on the initial and final

setting time. The explanation for these results lies on the

calorimetric curves and the cumulative heats of the two

combinations (Fig. 12). It can be seen that the initial

shoulder peak in SRPC/FA was occurred faster than that of

CSAC/ANH. This reflected an earlier consumption of

calcium sulfate and formation of the hydrates. Consequently

0

50

100

150

Initial

Final

Different percentages

Sett

ing

tim

e (S

eco

nd

s)

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a higher water demand caused final sets to reduce it all these

due to presence of ye'elimite in fly ash.CSAC/FGDG setting

times were higher than those of CSAC/ANH and SRPC/FA

probably due to the variability of the commercial product, in

combination with the presence of impurities that ultimately

affected its reactivity [25].

IV. CONCLUSIONS

Given the need for the utilization of alternative

cementitious systems to reduce the environmental impact

associated with Portland cement manufacture, alternative

cementitious systems of lower eCO2, when optimally

proportioned and based on calcium sulfoaluminate SRPC-

fly ash, may potentially offer environmental benefits. The

following conclusions are made according to the results of

this paper:

•The use of FA in SRPC in the presence of ye'elimite

promoted an earlier formation of a strong ettringite-rich

matrix, firmly accommodating FA particles with earlier

final sets. Both the FA particles and the formed hydrated

phases appeared to synergistically contribute to a dense

microstructure. Accumulated heat outputs and early

strengths reached higher values than those of the pure

CSAC/ANH and CSAC/FGDG combinations.

•The incorporation of anhydrite in the CSAC appeared to be

more mechanically beneficial than that of hemihydrate

form. In the hemihydrate-based systems, gypsum formation

occurred at very early hydration stages and this was

detectable by TG and XRD. Al(OH)3 quantities were

comparable at all CSAC systems as shown by TG. The use

of hemihydrate (FGDG) was found to accelerate the

formation of phases, based on the heat patterns.

V. ACKNOWLEDGEMENTS

The authors would like to thank the Civil and

Structural department, Annamalai University, Chidambaram

Tamil nadu. The authors acknowledge Neyveli Lignite

Corporation of India, Neyveli, Tamil Nadu, India for

providing the fly ash materials. The author wishes to

acknowledge the assistance rendered by Dr. C. Antany

Jayasekhar, professor and Head, Department of civil and

structural Engineering, Mr. S. Krishnan, Lecturer in

Mechanical Engineering, FEAT, Annamalai University and

Mr. A. Arulvanan, instructor in civil and structural

Engineering Annamalai University for their help in

recording the strength measurements and other discussions.

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